The present invention relates to piezoelectric ceramic compositions for use in piezoelectric ceramic elements, and particularly to piezoelectric ceramic compositions for use in materials for piezoelectric resonators and piezo vibrators, and piezoelectric elements for use in sensors, actuators, buzzers, etc.
Ceramic compositions comprising lead zirconate titanate (PZT) or lead titanate (PT) as main components are widely used as piezoelectric ceramic compositions for use in piezoelectric ceramic elements of piezoelectric ceramic vibrators, piezoelectric ceramic filters, piezoelectric buzzers, piezoelectric sensors and actuators. To improve the ceramic characteristics of the above compositions, traces of additives such as Mn, etc. are added, or a part of Pb is substituted with Sr or a composite oxide such as Pb(Mg1/3Nb2/3)O3 so as to obtain a piezoelectric ceramic having desired piezoelectric characteristics.
On the other hand, as for piezoelectric ceramic compositions containing no lead, (Na,K)NbO3, Bi4Ti3O12, etc. are known as ferroelectrics capable of exhibiting piezoelectricity, which, however, have not achieved sufficient properties in view of performance in practical use, and thus, are examined dominantly for use in sensors and actuators to be used under high temperature conditions.
The typical piezoelectric ceramic compositions such as lead zirconate titanate, etc., mainly used at present, contain Pb in large amounts which have adverse influences on the global environment. As more and more public attentions recently have been attracted to the environmental problems, the dumping of the waste of these materials induces serious problems from a view point of environmental protection. Under these circumstances, development of a material which contains no lead and has piezoelectric characteristics suitable for practical use is desired. However, at the present, there has been provided no piezoelectric ceramic composition that has piezoelectric characteristics higher than those of PZT type ceramics and PT type ceramics both of which contain Pb. Especially, a ceramic composition comprising barium titanate as a main component has been used in a restricted application such as Langevin type ultrasonic resonator or the like. However, ceramic compositions of this type are not suitable for practical use in general-purpose sensors and actuators. This is because, in case where such a composition is used for the above sensors and actuators, heating is needed to bond and set hard for junction, and because the piezoelectric ceramic composition can not be heated at a temperature higher than a Curie temperature at which its ferroelectricity is lost.
An object of the present invention is therefore to provide a piezoelectric ceramic composition which contains no lead and has piezoelectric characteristics suitable for practical use and thereby to solve the foregoing problems.
A piezoelectric ceramic composition according to the present invention has a complex perovskite structure represented by the formula:
(1xe2x88x92xxe2x88x92yxe2x88x92z)BaTiO3-x(Bi1/2Na1/2)TiO3-yCaTiO3-zBa(Zn1/3Nb2/3)O3xe2x80x83xe2x80x83(1)
and the composition comprises Ba, Ti, O, Bi, Na, Ca, Zn and Nb in the proportion satisfying the following conditions of x, y and z in the formula:
0.0 less than xxe2x89xa60.90,
0.0xe2x89xa6yxe2x89xa60.20;
and
0.0xe2x89xa6zxe2x89xa60.05,xe2x80x83xe2x80x83(2)
with the proviso that y and z are not zero at the same time (the condition of y=z=0 is excluded).
The piezoelectric ceramic composition of the present invention mentioned above contains no lead, and thus causes no environmental problem. In addition, it has a relatively large electromechanical coupling coefficient and therefore is suitably used for piezoelectric ceramic elements of piezoelectric ceramic filters, piezoelectric ceramic resonators, piezoelectric sensors, piezoelectric actuators, etc. Further, since the above piezoelectric ceramic composition has heat resistance(thermal stability) to heat of not lower than about 150xc2x0 C., it becomes possible to construct a piezoelectric element using adhesive having a setting temperature of 150xc2x0 C. In this regard, the above numerical range of x extrudes 0 and 1.0, because the single use of a known simple BatiO3 or (Bi1/2Na1/2) TiO3 is hard to provide a piezoelectric ceramic composition having heat resistance(thermal stability) sufficient for practical use. While the piezoelectric ceramic compositions of the present invention are represented by the formula (1), such compositions include not only compositions comprising independent phases of BaTiO3, (Bi1/2Na1/2)TiO3, CaTiO3 and Ba(Zn1/3Nb2/3)O3, but also compositions of the above formula in which, at least, the respective atoms, Ba, Ti, O, Bi, Na, Ca, Zn and Nb are contained in the proportion satisfying the above conditions of numerical ranges (2) and in which these atoms constitutes a perovskite structure represented by ABO3.
It is preferable that the piezoelectric ceramic composition comprises MnO2 as a sub-component at a content of 0.01 to 1.0 wt. %. This is because, out of piezoelectric characteristics, particularly a mechanical quality factor (the acuteness of mechanical vibration at a resonance frequency) can be improved.
Also, it is preferable that the piezoelectric ceramic composition comprises a rare earth oxide such as Y2O3 as a sub-component at a content of 0.01 to 1.0 wt. %. This is because the time dependent change(a change with the passage of time) in capacity can be decreased, which leads to an improvement on the stability of piezoelectric characteristics with time.
The addition of the sub-components, that is, MnO2 and a rare earth oxide such as Y2O3 is effective to further improve the piezoelectric characteristics, and it is preferable that any of such sub-components should be added in an amount of not more than 1.0wt. %, because the addition of not less than 1.0wt. % of such a sub-component has a danger of degrading the characteristics as piezoelectric ceramics.